1. Technical Field
The present disclosure relates to RF isolation switch circuits.
2. Background Information
There are many places in high frequency Radio Frequency (RF) circuits where a source of an RF signal is to be selectably either coupled to a load or is to be isolated from the load. The term “RF switch” is sometimes used to refer to a circuit that can be controlled either to couple the RF source to the load or to disconnect and isolate the RF source from the load. One location where such an RF switch is used is in a radio transmitter in the signal path of an RF signal between the transmitter balun and the transmitter driver amplifier. An RF transmit signal as supplied by the balun is either to be coupled to an input lead of the driver amplifier or is to be is to be blocked from reaching the input lead of the driver amplifier. Conventionally the RF switch is realized as a single N-channel field effect transistor. This transistor is typically made small enough that it does not load the balun output with excessive parasitic capacitance when the transistor is controlled to be off. The transistor is, however, typically made large enough to have an adequately low source-to-drain on resistance when it is controlled to be on. If the RF signal voltage swing as output by the balun is high, then a thick gate dielectric N-channel transistor is used. Alternatively, a T-switch configuration of three N-channel transistors may be used. Although these conventional RF switch circuits operate satisfactorily well in many instances, the RF switches do introduce unwanted parasitic capacitances and do degrade circuit linearity. Improvements in the operation of RF switches and in the operation of the overall transmitter circuit are sought.